Method and device for producing slabs of steel

ABSTRACT

The invention relates to a process for producing slabs from steel, in which the strand leaves a permanent mold with liquid melt enclosed by the strand shell and, in a downstream strand guiding assembly, the gap between guide rollers mounted in stands is set infinitely variably by adjusting elements connecting lower and upper frames, characterized by the following steps:  
     a) the gap (s) is changed by an oscillation about a predeteminable center line (c) of the gap in such a way that the dynamic influences on the guide rollers are negligible,  
     b) the amplitude (A) of the gap oscillation is set to a value which does not induce any plastic deformation of the strand shell,  
     c) the current gap (s) is recorded,  
     d) at the same time, the actuating force (F) of the adjusting elements and the amplitude (A) of the actuating force are determined and  
     e) with increasing amplitude (A) of the actuating force (F), the gap (s) is set to a predeterminable value and/or is pressure-controlled by means of at least one adjusting element.

DESCRIPTION

[0001] The invention relates to a process for producing slabs fromsteel, in which the strand leaves a permanent mold with liquid meltenclosed by the strand shell and, in a downstream strand guidingassembly, the gap between guide rollers mounted in stands is setinfinitely variably by adjusting elements connecting lower and upperframes, and relates to an associated apparatus for this.

[0002] DE 26 12 094 C2 discloses an apparatus for changing the distancebetween parts of a frame or stand of a strand guiding assembly lyingopposite one another in pairs and connected by tie rods, in which busheswhich can be turned with the aid of pressure cylinders are provided. Themovable frame parts are connected by pressure cylinders, exchangeablespacers being insertable between the movable frame part and the innerbushes for the purpose of setting a predeterminable roller spacing. Onthis embodiment, an infinitely variable setting of the spacing betweenthe guide rollers can also be carried out.

[0003] In a disadvantageous way, the adjustment of the gap by theturning of the bushes is possible only over a very limited distance. Inaddition, considerable mechanical wear must be expected during theadjusting operation. With these known hydraulic clamping cylinders, itis not possible for the clamping force to be deduced, since part of theclamping force is absorbed by the so-called spacers.

[0004] U.S. Pat. No. 3,891,025 discloses continuous casting stands whichare hydraulically adjustable and the gap of which is recorded byposition sensors and a servo unit can be set.

[0005] The essential object of the subject matter of this patent ismerely to apply adequate pressing force, or set the gap, fortransporting the strand.

[0006] DE-A-24 44 443 discloses a process for continuously casting asteel melt in which the change in thickness of the casting is determinedand compared with a specific reference value, in order in this way tocontrol the drawing rate and/or the amount of secondary cooling water.

[0007] Practice has shown that such a method of detecting the lowestpoint of the liquid crater can be used only in the case of ageometrically ideal installation and a quite specific casting rate andcooling. In the hostile conditions of a metallurgical plant, however, aninstallation cannot be set up exactly with respect to the gap, or elsethermal deformations occur in the segments or the installation operatesin an inexact way, with the consequence that the changes in thicknessdetermined are subject to considerable variations, in particular in theregion of the lowest point of the liquid crater.

[0008] Cognizant of the difficulties mentioned above, the object of theinvention is to provide a process and an apparatus with which the gapcan be set exactly over the entire strand guiding assembly by simplemeans and, in addition, the current position of the lowest point of theliquid crater within the slab can be determined. Furthermore, whilebeing of a simple construction, the apparatus is to be capable ofreliably guiding the cold strand.

[0009] The invention achieves the object by the characterizing featuresof process claim 1 and apparatus claims 6 and 8.

[0010] According to the invention, the gap is changed by an oscillationabout a predeterminable center line of the slab thickness aimed for. Inthis case, an oscillation value which keeps to a negligible level thedynamic influences on the strand shell, which is still relatively thinafter leaving the mold, is chosen. The amplitude of the oscillating gapis set to a value which prevents plastic deformation of the strandshell.

[0011] The current value of the gap is recorded by means of distancemeasuring elements and is fed to a computer. At the same time, theactuating force of the adjusting elements for the infinitely variablechanging of the gap is determined and likewise fed to the computer. Bymeans of a computing program, the amplitude is monitored and, when theamplitude of the actuating force increases, the gap is set to apredeterminable value and/or the gap between the guide rollers ispressure-controlled by means of one of the adjusting elements settingthe gap in an infinitely variable manner.

[0012] The amplitude of the actuating force is in this case a measure ofthe solidifying through of the strand. That is to say, a relativelysmall amplitude of the actuating force is encountered when the strandshell is still thin and there is a large liquid crater. The amplitudereaches its greatest value when the strand is solidified through.

[0013] Consequently, recording the amplitude of the actuating forceprovides a reliable measure for recording the current position of thelowest point of the liquid crater and carrying out a dynamic softreduction.

[0014] The computer also establishes a relationship between the gap andthe actuating force. It has been found in this case that, if the gapdeviates from its optimum value, the following situation arises:

[0015] if the gap is smaller than the optimum, the edge pressure of theslab increases, with the consequence that the actuating force increases

[0016] if the gap is larger than the optimum, no edge pressure occursand the strand bulges, the actuating force assuming a lower overallvalue.

[0017] In the case of quasi-static measurement, in first approximationthis can be represented by two simple curves F₁ and F₂, which representsoverall the form of an angle with two sides. At the optimum gap, theoptimum pressure distribution over this strand shell and the liquidcrater enclosed by it is also to be encountered.

[0018] Recording the current actuating force allows the optimum gap tobe set by detecting from the oscillation whether the trend away from theoptimum gap is toward the larger or smaller gap, in order then to takespecific measures to counteract this.

[0019] In the case of dynamic measurement, the actuating force F behaveswith respect to the gap s in the form of a hysteresis curve. Thedeformation work of a segment during the stroke, i.e. the area withinthe hysteresis curve, can be calculated by evaluation software and thestrand consistency can be deduced. The hysteresis curve has a relativelysmall area overall when the shell is still thin and the crater isrelatively large. The hysteresis curve has a relatively large area whenthe shell is continuing to grow and the crater volume is decreasing. Thehysteresis assumes a particularly slender form when the strand hassolidified right through.

[0020] The invention achieves an optimization of the productionperformance from qualitative and quantitative aspects, to be precisewith respect to qualitative optimization by a soft reduction which isalways carried out optimally (seen locally, dynamic soft reduction) andwith respect to quantitative optimization of the production performanceby the possibility of being able to maximize utilization of the machinelength, with high operational reliability at the same time.

[0021] Moreover, if displacement-controlled hydraulics are used, nofurther mechanical components are required.

[0022] In addition, any so-called thermal tracking software there may beis considerably improved in its accuracy.

[0023] An example of the invention is represented in the attacheddrawing, in which:

[0024]FIG. 1 shows the diagram of the continuous casting installation,

[0025]FIG. 2 shows the dependence of the gap or the actuating force overtime,

[0026]FIG. 3 shows the dependence of the actuating force over the gap,

[0027]FIG. 4 shows the formation of the hysteresis curve and

[0028]FIG. 5 shows stands in various operating states.

[0029]FIG. 1 shows, in the upper part of the image, the diagram of acontinuous casting installation with a permanent mold 11, at the mouthof which a slab B emerges and is guided by stands 21.1 to 21.5. In theslab, the strand shell of which gradually solidifies, there is a craterS up to a lowest point Ss For the sake of simplicity, adjusting elements31 are represented only in the case of the stand 21.4.

[0030] Presented in the lower part of the image is the diagram of astand 21, which has an upper frame 22 and a lower frame 23, whichdetermine by means of adjusting elements 31 the gap between the guiderollers 24 arranged on them. One of the guide rollers is a drive roller25, the function of which will be described in further detail in FIG. 5.

[0031] The adjusting elements have a tie rod 32, which as a rule isfastened in the lower frame 23 and has at its opposite end a piston 33,which is guided in a cylinder 34. The individual stands 21 have at leastfour adjusting elements 31, the cylinders 34 of which are in connectionwith an actuator 35.

[0032] In the left-hand part of the diagram, the adjusting element 31 isequipped with a distance-measuring element, which is in connection witha distance-measuring pick-up, which is connected in terms of measuringtechnology to a computer.

[0033] In the right-hand part of the diagram, the cylinder 34 isequipped with a pressure-measuring element 43, which is connected to apressure pick-up 44, which is likewise connected in terms of measuringtechnology to the computer. The computer 45 cooperates in control termswith the actuator 36.

[0034] In addition, the actuator is connected to an oscillator.

[0035] In FIG. 2, in the upper part of the image, the gap is plottedover time. By means of an oscillator, the gap is changed by the slabthickness aimed for (center line c). In the present case, it is asinusoidal oscillation. However, other modes of oscillation are alsopossible and envisaged.

[0036] In the lower part of the image, the actuating force F is plottedover time. In the left-hand part of the image, the actuating force has arelatively small amplitude. In the right-hand part, the amplitude of theactuating force has increased distinctly.

[0037] In FIG. 3, the dependence of the actuating force over the gap isrepresented. It is evident that, in first approximation, two curves, orin the greatest simplification two straight lines, to be preciseF₁=a−m₁·s and F₂=b−m₂·s, can be represented by means of a computer.Since the two curves have different slopes, they intersect at a point P.

[0038] In a further approximation, the actuating force F/gap S [sic]shows a hysteresis which has substantially the form of an angle with twosides, with an apex point P. The optimum gap is expected in the regionof the point.

[0039] Should it become evident in the evaluation during operation thatthe hysteresis curve is migrating along one side F₁ or F₂, measures areto be taken to the effect that both sides are of approximately the samesize and that their point of intersection and the break point of thehysteresis are in the region of the point P, in other words close to theoptimum of the gap.

[0040] Should the image evaluation show that the hysteresis no longerhas a break point and consequently has migrated out along one side ofthe angle F₁ or F₂, measures are to be taken in the form and directionof the gap in order that the hysteresis is as uniform as possible onboth sides of the point P.

[0041] In FIG. 4, the dependence of the actuating force over the gap hasbeen refined even further. In dependence of the size of the crater, thehysteresis develops from type a through type β to solidified-throughtype γ.

[0042] Thus, the crater of type a has a thin shell with a crater of lowviscosity, type β has a distinctly thicker shell and at the same time acrater with high viscosity and type γ has altogether solidified through.

[0043] The image representations presented here show a uniformdistribution for the hystereses and consequently the optimum gap, eithers_(a) or else s_(β).

[0044] The actual forms of the hystereses detectable during operationconsequently allow the deviation from the optimum gap to be detected andthe correct measures to be adapted in dependence on the degree anddirection of the adjustment of the gap. Furthermore, conclusions can bedrawn as to the degree of solidification.

[0045]FIG. 5 shows a stand in three different operating states. The itemnumbers correspond to those already presented in the images above. Inthe upper part of the image is normal casting operation, in which aposition control is carried out on all cylinders. In the presentexample, a drivable guide roller is provided at the stand inlet on theupper frame.

[0046] In the middle part, operation when the strand has solidifiedthrough is represented. Here, the cylinders for the adjusting elementsarranged in the region of the drivable guide roller arepressure-controlled and the cylinders represented downstream withrespect to the strand are position-controlled.

[0047] In the lower part of FIG. 5, for transporting the cold strand,the upper frame of the stand is inclined in such a way that the driveroller has direct contact with the cold strand by means of the adjustingelements arranged in the vicinity of said roller, by pressure control ofthe cylinders, and the cylinders of the adjusting elements which arearranged away from the drive roller are position-controlled. In thiscase, their position is set such that during the transport of the coldstrand they do not have any contact with the latter.

1. Process for producing slabs from steel, in which the strand leaves apermanent mold with liquid melt enclosed by the strand shell and, in adownstream strand guiding assembly, the gap between guide rollersmounted in stands is set infinitely variably by adjusting elementsconnecting lower and upper frames, characterized by the following steps:a) the gap (s) is changed by an oscillation about a predeteminablecenter line (c) of the gap in such a way that the dynamic influences onthe guide rollers are negligible, b) the amplitude (A) of the gaposcillation is set to a value which does not induce any plasticdeformation of the strand shell, c) the current gap (s) is recorded, d)at the same time, the actuating force (F) of the adjusting elements andthe amplitude (A) of the actuating force are determined and e) withincreasing amplitude (A) of the actuating force (F), the gap (s) is setto a predeterminable value and/or is pressure-controlled by means of atleast one adjusting element.
 2. Process according to claim 1,characterized in that the frequency (f) of the gap oscillation is=0.05to 5.0 Hertz.
 3. Process according to claim 1, characterized in that thecurrent actuating force (F) is recorded in a computer-aided manner andpreprocessed in such a way that, in first approximation, the actuatingforce (F) behaves in dependence on the actual gap (s) like two curves:F₁=a−m₁·s F₂=b−m₂·s which have the form of an angle with two sides ofdifferent slope, with an apex point (P), and in that the gap (s) is setas a function of the relationship F=f (s) in such a way that theproportions of the sides F₁ and F₂ are kept substantially at the samesize.
 4. Process according to claim 3, characterized in that the oneside F₁=a−m₁·s corresponds to a gap which is smaller and the second sideF₂=b−m₂·s corresponds to a gap which is larger than the optimum gap (s)at the apex point (P) and in that the degree and direction of theadjustment of the gap (s) is adapted as a function of the relationshipF=f (s).
 5. Process according to claim 4 or 3, characterized in that thevariation of the actuating force (F) has in second approximation theform of a hysteresis, in that the extent of the actuating force (F) withrespect to an associated gap (s) [lacuna] as a measure of the viscosityof the liquid crater in the slab and in that, in dependence on theviscosity found, conclusions are drawn as to the position of the lowestpoint of the liquid crater and the gap adjustment is adapted. 6.Continuous casting installation for producing slabs from steel, having apermanent mold and downstream strand guiding assembly, which has standswith lower and upper frames on which there are provided guide rollers,the gap between which can be set infinitely variably by adjustingelements connecting the frames, for carrying out the process accordingto claim 1, characterized in that distance sensors (42) are provided,with which the gap (s) between the guide rollers (24) can be recorded,in that the distance sensors (42) are in connection with a computer(45), which is connected to an actuator (35) by which the adjustingelements (31) can be operated in a pressure- and/ordistance-controllable manner for gap setting and in that an oscillator(46) is provided, by which the adjusting elements (31) can be induced toundergo oscillation outside the resonant vibration with respect to thestrand stands (21).
 7. Continuous casting installation according toclaim 6, characterized in that the distance sensors (42) have measuringelements (41) which are connected directly to the adjusting element(31), in particular in the case of hydraulic adjusting elements areconnected to the adjusting piston (33).
 8. Stand of a strand guidingassembly which is arranged downstream of a permanent mold in acontinuous casting installation for producing slabs from steel, whichstand has lower and upper frames provided with guide rollers which canbe set in their distance with respect to each other by pressure- and/ordistance-controlled adjusting elements to a gap through which a coldstrand can also be transported, characterized in that one of the outerguide rollers (24) of the upper frame (22) can be driven, in that theadjusting elements (31) assigned to the drivable guide roller (25) canbe brought into connection with the computer (45) in terms of controltechnology via pressure-control devices (43, 44) and the other adjustingelements (31) can be brought into connection with said computer (45) interms of control technology via position-control devices (41, 42). 9.Continuous casting installation according to claim 8, characterized inthat the adjusting elements (31) are mounted in the upper and lowerframes (22, 23) in such a way that the frames (22, 23) can be adjustedat an inclination with respect to each other that the frames (22, 23)can be adjusted at an inclination with respect to each other [sic], thegreater gap opening facing away from the drive rollers (25).